Thermally compensatable tunable cavity



Jan. 14, 1964 H. SHAPIRO ETAL 3,118,120

THERMALLY COMPENSATABLE TUNABLE CAVITY Filed May 51, 1960 IN VE NTORS 454/27 .swa/uea United States Patent 3,113,120 THERE/EARLY GMPENSATAELETUNABELE QAVETY Help, Shapiro, New Rochelle, and Robert F. Romero,llaznaroneclr, N.Y., assignors, by assignments, to lunch Systems, mo,Beverly Hills, Califi, a corpo- Maryland Filed May 31, 196%, Ser. No.32,992 2 Claims. (@l. 333-433) This invention relates to the art ofresonant units, more particularly of the temperature compensatabletunable cavity type for high frequency transmission.

As conducive to an understanding of the invention, it is noted thatwhere a resonant unit is of the type such as illustratively shown inPatent No. 2,918,636, dated December 22, 1959, that includes an anodecavity which is tunable by moving a plunger controlled by rods extendingfrom the cavity, since the effective length of the anode cavitysubstantially deter-mines the operating frequency of the device, whereonce the plunger has been set by movement of its control rods, changesin ambient and/ or operating temperatures cause a change in theefiective length of the anode cavity, the operating frequency of thedevice will be varied.

Where attempts have been made to make the resonant unit temperaturestable by constructing the unit with materials with low thermalexpansion coefficients, the high cost of such materials renders theunits impracticable for ordinary commercial purposes. Furthermore, sincematerials suitable for resonant units that have a negative thermalcoefficient of expansion are not available, it has not been possibleintentionally to overcompensate a resonant unit to take care ofimperfect temperature compensation elsewhere in elements coupled to theresonant unit.

it is accordingly among the objects of the invention to provide aresonant unit that may readily be fabricated and is strong and sturdyand is not likely to become deranged, and which has an anode cavity thatmay readily be tuned to a desired operating frequency, and which willautomatically adjust the length of the anode cavity with increases inambient temperature to compensate for thermal variations in thematerials used in the construction of the resonant unit or relatedunits.

According to the invention, the resonant unit cornprises a cavity inwhich a plunger is slidably mounted and controlled by rods extendingfrom one end of the cavity, the plunger position substantiallydetermining the resonant 0 frequency of said cavity. The rods which arelocked in fixed position when the cavity is tuned are of material thathas a suitable thermal coefiicient of linear expansion relative to thatof the cavity, so that with an increase in ambient temperature, forexample, the resonant frequency can be decreased, decreased or heldsubstantially constant.

in the accompanying drawings in which is shown one of various possibleembodiments of the several features of the invention,

The single FIGURE is a fragmentary longitudinal sectional view of aresonant unit according to the invention, with parts broken away.

Referring now to the drawings, the temperature compensatable deviceshown to illustrate the invention is of the type shown in Patent No.2,918,636, the description and drawings of which are included herein byreference for a more complete showing of the environment in which thepresent invention is incorporated, and the unit herein shown will omy bedescribed to the extent necessary for an understanding of the presentinvention. Thus iii) ilhjzh Patented Jan. 14, 196

such device comprises a metal sleeve 10 which defines the outer wall ofthe anode cavity 11.

The lower end 41 of sleeve 10 is seated in a circular recess 42 in abottom plate and is at'tixed in said recess as by soldering. The plate'43 has a central opening 44 coaxial with sleeve 10 and a pair ofdiametrically aligned openings 45, the rear edge of plate 43 having amounting flange 47. Aflixed to plate 43 is a disc 51 which has anoutstanding axial hub 52 coaxial with the opening 44 in plate 43 and apair of openings 5! aligned with opening 45.

Extending through hub 52 into sleeve 10 and aifixed to the hub as at 53is a metal sleeve 54 which defines the inner wall of the anode cavity 11and the outer wall of a grid-cathode cavity 55 more fully described insaid Patent No. 2,918,636.

The inner end 58 of sleeve 54 mounts a contact ring 59 which has aplurality of resilient contact fingers 61 designed to make an effectiveelectrical and mechanical connection with the grid ring 62 of tube 34-.

Slidably mounted on sleeve 54 in sleeve 10 is a metal plunger 64 bymeans of which the anode cavity 11 is tuned. The plunger 64 on its innerand outer peripheries has resilient contact fingers 6-5 which engage theopposed surfaces of sleeves it and 54 to provide a good electricalconnection and also serve to retain the plunger 64 in set position, thevolume of sleeve 11 between plunger '64 and disc 17 defining the activeportion of the anode cavity.

To adjust the position of plunger 64 to tune the anode cavity 11, asleeve 181 is slidably positioned in each of the aligned pairs ofopenings 45, Si in bottom plate 43 and disc 51. Each of the sleeves 181extends through a hub 182 secured to the outer surface of disc 51 and issecured in any desired position by set screws 183.

A rod 66 extends through each of the sleeves 181 and is locked indesired position as by set screws 13 extending through the lower end ofthe associated sleeve 181.

Thus, referring to the drawing, with the sleeves 181 pushed upwardly asfar as possible and locked in position by set screws 183, the effectivelength of the rods 66 will be from the center line of set screws 184 tothe bottom of the plunger 64. With the sleeves 131 moved downwardly, itis apparent that the distance between the set screws 184- and the bottomof the plunger will be increased, thereby materially increasing theeffective length of the rods 66 and hence increasing their eifectivenessas temperature compensating elements.

According to the invention, the rods 66' connected to plunger 64 are ofmaterial such as to have a coetlicient of linear thermal expansion withrespect to the sleeve 16 as to effect a movement of the plunger 64 withchanges in ambient temperature that will increase, decrease or holdsubstantially constant the resonant frequency of the cavity 19.

Thus, assuming that the cavity is of brass, which has a coefiicient oflinear thermal expansion of approximately 17.7 to 212x10" per degreecentigrade, with a temperature range of from 25 to 300 degrees C., therods 66 could be of polystyrene, which has a coeflicient of linearthermal expansion of approximately l0 per degree 0., orphenol-formaldehyde, which has a coefficient of 3G to 40x10 per degreeC.

In both illustrations, the coeficient of linear thermal expansion of therods 66 is greater than that of the sleeve 10. Hence, based upon thedistance of the plunger 64 from the location at which the rods aresecured by set screws 184, which depends on the length of the rods andthe material of such rods, the position of the plunger in the cavity illwill vary with changes in ambient tempera- '5 3 ture so that theresonant frequency of the cavity may be increased, decreased or heldsubstantially constant.

in practice, the type of material to be used for the rod is determinedempirically. based on the efiective length of the rods 65 by checkingthe resonant frequency at difierent temperatures and with different rodmaterials, and selecting the particular rods which give the desiredcompensation.

The specific mathematical relations which illustrate the compensationare as follows: Referring to the drawing, let

K1 =coeificient of linear thermal expansion of sleeve 10 K2=coefiicientof linear thermal expansion of sleeve 54 K3=coefiicient of linearthermal expansion of plunger 64 K4=coetficient of linear thermalexpansion of plate 43 K5=coefiicient of linear thermal expansion ofblock 182 K6=coefiicient of linear thermal expansion of rods 66 Dl=innerdiameter of sleeve 10 D2=outer diameter of sleeve 54 C =eifectivecapacity of plate-grid section of tube 34 L length of sleeve 14 frompoint of contact of plate of tube 34 to plate 43 A=thickness of plate 43B=distance from top face of block 73 to center line of set screws 184W=thickness of plunger 64 X =distance from point of contact of sleeveltl to plate of tube 34 to top of plunger 64- C=distance from bottomface of plunger 64- to center line of set screws 184 V=velocity of lightf resonant frequency of cavity 11 M=mode of resonance (n O, quarterwave; n= 1, three quarter wave, etc.)

Ignoring the detuning sheet of the output '75, the resonant frequency ofthe plate-grid coaxial cavity 11 is derived as follows:

The characteristic impedance of a coaxial cavity is:

The Equation 3 shows the dependence of the resonant frequency f on theplunger position X; the two diarncters D1, D2, and the effective tubecapacity C Thus, if the plunger'position X can be made'to vary asdesired, as the ambient and/or operating temperatures change, theresonant frequency can be made to remain stationary or vary in aprescribed manner.

. Referring to the drawing, plunger position X is given by: t

' 'X=L+A+B-C W For any temperature T, the plunger position X is:

in the Equation 5 it is understood that L, A, B, C and W are measured attemperature T degrees. The change in plunger position X caused by achange in temperature from T to T degrees is given by: r

Referring to Equation 6, it is clear that by a proper (a) L6=% (b) Ks(c) K6 With the relatively simple constructions above described, theeffective length of the rods s5 may be appreciably changed independentlyof the position of the plunger to provide accurate compensation withoutchanging the rod material and it is apparent that with tunable cavitiesof given physical characteristics, once the physical characteristics ofthe rods have been determined, production runs of such cavities may bemade which will have the desired compensation.

As many changes could be made in the above equipment, and manyapparently widely different embodiments of th s invention could be madewithout departing from the scope of the claims, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

Having thus described our invention, what we claim as new and desire tosecure by Letters Patent of the United States is:

l. A resonant cavity tunable over a broad band of frequencies,comprising a cylindrical sleeve having at least one end wall, a plungerslidaoly mounted in said sleeve, at least one rod secured at one end tosaid plunger and extending parallel to the longitudinal axis of saidsleeve through said end wall, said rod having a thermal coefiicient oflinear expansion differing from that of said sleeve to provide apredetermined variation of the position of said plunger in said sleevewith changes in temperature, a member secured to the outer surface ofsaid end Wall adjacent said rod, a movable sleeve extending through saidend wall and said member, said rod extending through said movablesleeve, means extending through said member to lock said movable sleevein a predetermined position, and means extending through the outer endof said movable sleeve to lock said rod in a predetermined position,whereby the effective length of said rod will extend from said secondlocking means to said plunger.

2. A device of the character described tunable over a broad band offrequencies where the maximum frequency is many times the minimumfrequency, comprising three coaxial metal sleeves, the outermost sleevebeing shorter than the other two sleeves, said outermost sleeve and thesecond sleeve adjacent thereto defining an anode cavity, said secondsleeve and the third sleeve defining a gridcathode cavity, means to tunesaid cavities, said tuning means for at least the anode cavitycomprising a plunger slidably mounted in said cavity between theoutermost sleeve and the second sleeve adjacent thereto, said anodecavity having an end wall, a member secured to the outer surface of saidend wall, a movable sleeve extending through said end wall and saidmember, a rod secured. at one end to said plunger and extending parallelto the longitudinal axis of said anode cavity through said movablesleeve, means extending through said member to lock said movable sleevein a predetermined position, means extending through the outer end ofsaid movable sleeve to lock said rod in a predetermined position,whereby the effective'length of said rod vdl-l extend from said secondlocking means to said plunger, said rod having a thermal coefficient oflinear expansion differing from that of said outermost sleeve to providea predetermined variation of the position of said plunger in said anodecavity with changes in temperature, said second sleeve having anelongated longitudinal slot therethrough in the portion thereofextending beyond the outenrnost sleeve, a slidable member mounted on theportion of said second sleeve extending beyond the outermost sleeve, aninput carried by said slidable member, a contact member electricallyconnected to said input, carried by said slidable member and insulatedtherefrom and from said second sleeve, said contact member extendingthrough said slot and resiliently and slidably engaging said thirdsleeve to feed the input signal into the grid-cathode cavity, the lowestoperating frequency of said device being determined by the effectivemaximum length of the anode cavity, the far end of the slot in thesecond sleeve being at a distance from the inner end of the secondsleeve substantially equal to an odd multiple of one quarter Wave lengthwhich is at least the next larger odd multiple of the number of quarterWave lengths used in the anode circuit.

References Cited in the file of this patent UNITED STATES PATENTS2,109,880 DOW Mar. 1, 1938 2,456,770 Dearing Dec. 21, 1948 2,790,151Riblet Apr. 23, 1957 2,918,636 Shapiro Dec. 22, 1959

1. A RESONANT CAVITY TUNABLE OVER A BROAD BAND OF FREQUENCIES,COMPRISING A CYLINDRICAL SLEEVE HAVING AT LEAST ONE END WALL, A PLUNGERSLIDABLY MOUNTED IN SAID SLEEVE, AT LEAST ONE ROD SECURED AT ONE END TOSAID PLUNGER AND EXTENDING PARALLEL TO THE LONGITUDINAL AXIS OF SAIDSLEEVE THROUGH SAID END WALL, SAID ROD HAVING A THERMAL COEFFICIENT OFLINEAR EXPANSION DIFFERING FROM THAT OF SAID SLEEVE TO PROVIDE APREDETERMINED VARIATION OF THE POSITION OF SAID PLUNGER IN SAID SLEEVEWITH CHANGES IN TEMPERATURE, A MEMBER SECURED TO THE OUTER SURFACE OFSAID END WALL ADJACENT SAID ROD, A MOVABLE SLEEVE EXTENDING THROUGH SAIDEND WALL AND SAID MEMBER, SAID ROD EXTENDING THROUGH SAID MOVABLESLEEVE, MEANS EXTENDING THROUGH SAID MEMBER TO LOCK SAID MOVABLE SLEEVEIN A PREDETERMINED POSITION, AND MEANS EXTENDING THROUGH THE OUTER ENDOF SAID MOVABLE SLEEVE TO LOCK SAID ROD IN A PREDETERMINED POSITION,WHEREBY THE EFFECTIVE LENGTH OF SAID ROD WILL EXTEND FROM SAID SECONDLOCKING MEANS TO SAID PLUNGER.